In the past few years, the growing demand for low voltage, low power, low cost, high performance mobile communications equipment has changed the way wireless receivers are designed. BiCMOS technology has become a practical contender for use in receiver design, especially because it lends itself to easier integration with digital integrated circuits (ICs), as well as analog circuits. However, usage of submicron ceramic metal-oxide semi-conductor (CMOS) technologies imposes an upper limit on the supply voltages, therefore it is important to focus on low voltage design when designing RF CMOS circuits. Low voltage designs also reduce the average current drain for digital integrated circuits. However, this may not always have the same application to analog circuitry. Moreover, communication products are becoming increasingly complex. By improving on existing circuitry and systems, a reduction in power consumption will enable the operation of ever more complicated communication products without sacrificing product battery life performance. Most modem mixers used in current wireless receivers are based on the classical Gilbert cell. However, one improvement that can always be made to present Gilbert cell designs is a reduction in current drain.
As seen in prior art FIG. 1, a typical Gilbert cell mixer 100 generally located at the front-end of a zero intermediate frequency (ZIF) receiver includes an input 101 that typically feeds into a low noise amplifier (LNA) 103. The LNA 103 is used to amplify the input signal without generating an inordinate amount of noise and distortion signals. This enables the receiver to maintain a significantly high signal-to-noise ratio (SNR). The output of the LNA 103 is fed in a differential fashion to the inputs of transconductance amplifier 105 and transconductance amplifier 107. Both transconductance amplifiers 105, 107 are linear gain stages that provide a high degree of gain while isolating an in-phase (I) and quadrature (Q) local oscillator (LO) mixer input signals 113, 117 from interfering with the output of LNA 103. Without these stages the LO signals 113, 117 can degrade receiver performance by introducing imbalance between I signal 121, Q signal 123, and by creating DC offsets. As is known in the art, the transconductance amplifier is used to convert the RF input voltage into a current. The output of the transconductance amplifiers 105, 107 are then directed to an in-phase LO driven Gilbert cell mixer 109 and a quadrature LO driven Gilbert mixer cell 111. They work with a local oscillator input signals 113, 117 to mix each respective RF input signal from transconductance stages 105, 107 to produce in-phase (I) and quadrature (Q) output signals 121, 123. The local oscillator signals 113 and 117 for the first Gilbert cell mixer 109 and second Gilbert cell mixer 111 are generated by an RF oscillator operating at two times (2×) the oscillator frequency. This is accomplished by operating a voltage controlled oscillator (VCO) 115 whose output duty cycle is 50% with a divide by two circuit producing two local oscillator (LO) signals for both in-phase (I) and quadrature (Q) inputs 113 (LOi) and 117 (LOq). The LOi and LOq are then mixed with the input RF signal where they are mixed down to a lower intermediate frequency (IF) or baseband to provide an in-phase (I) output signal 121 and a quadrature (Q) output signal 123.
One major drawback associated with this type of mixer topology is the high current drain required in the transconductance stages 105, 107. When placed on-chip these stages operate with a high degree of isolation and linearity, however this comes at the cost of high power consumption or current drain. Obviously this plays a critical role by ultimately shortening the operating time of any portable battery operated device. If the output of the LNA 103 were connected directly to the input of each respective mixer by removing the transconductance stages, there would be an inadequate amount of isolation between the various mixing signals and would be detrimental to receiver performance. However, this would reduce the current drain substantially. Thus, the need exists to innovate an alternative mixer topology that reduces the current drain by removing the transconductance stages while maintaining a high degree of in-phase and quadrature IF isolation and linearity.